This invention relates to improved hot-melt adhesive compositions effective in binding polyolefins to themselves and to other materials which are based upon a poly(vinylalkylether) and, more particularly, to hot-melt adhesive compositions which strongly bind polyolefins, in particular polyethylene, polypropylene, and ethylene-propylene copolymers, to themselves and other substrates, and which are formulated from a poly(vinylmethylether), a thermoplastic resin, and optionally, a tackifier resin.
Hot-melt adhesives produce a bond between two materials by cooling of the molten adhesive which is applied to the materials in a melted state. They are distinguishable from those products which require crosslinking or other chemical reactions to achieve ultimate bond strength and from those materials that require loss of solvent or water to achieve the bond strength. Prior to heating, the hot-melt adhesives are solids that can be provided in bulk or in pelletized or rod form for ease of handling. Upon heating, the hot-melt adhesive composition melts rather sharply and flows readily for application to a substrate. Since hot-melt adhesives are thermoplastic rather than thermosetting, and are thus remeltable, they can be applied to a first substrate and later remelted to form a hot-melt bond with a second substrate. Hot-melt adhesives can have considerable advantage over other types of adhesives for a number of uses and have a significant commercial importance.
There are a variety of uses for hot-melt adhesives which are out in the market place. However, while one hot-melt adhesive may be used for bonding in a particular use or application, the same adhesive may be completely unsuitable for other uses or applications. While many satisfactory hot-melt adhesives are available to the art, the art is constantly seeking new compositions having superior performance, lower cost, and/or stronger bonding of the substrate.
Polyethylene, polypropylene, and ethylene-propylene copolymers are widely used in many applications which require adhering these plastics to themselves as well as to other materials. However, adhesives do not form strong bonds to untreated polyolefin surfaces. The difficulties encountered in developing adequate adhesion to polyethylene and polypropylene can be attributed in large part to the non-porous and non-polar characteristics of these materials. In addition, the presence of surface materials such as mold release agents, slip agents, antioxidants, polymerization impurities, and low-molecular-weight, surface polymer further deteriorates surface conditions for adhesion. Various surface preparation procedures, including etching with sodium dichromate-sulfuric acid solution, treating with flame, corona discharge, electron beam, laser light, ultraviolet radiation, hot chlorinated solvent, and applying chemically reactive primers have been developed to afford a polyolefin surface that is more receptive to conventional adhesives. However, such pretreatment adds significant cost to a finished assembly and can reduce the economic incentive to use polyethylene or polypropylene in place of more expensive plastics. Further, some surface treatment procedures, e.g., corona discharge, are only applicable to thin, flat surfaces and cannot be used in many existing applications.
An adhesive which strongly bonds untreated polyethylene, polypropylene, and ethylene-propylene copolymers would represent a significant achievement and present opportunities to not only market a new family of adhesives, but also open many new markets for such polyolefins in the areas of automobiles, appliances, fabrics, etc.
Compatibility of adhesive and substrate is critical if a strong bond is to be formed. An estimate of compatibility can be obtained from the solubility parameters (.delta.) and critical surface tensions (.gamma..sub.c) of the materials of interest. Generally, a good correlation exists between solubility parameter and surface tension for polymers. To form a strong bond, the adhesive should readily wet the surface of the substrate. Polyethylene and polypropylene have .gamma..sub.c values of 31 and 29 dynes/cm and .delta. values of 7.7-9.2, while typical adhesives have considerably higher values so little spreading and wetting occurs, giving poor adhesion using these adhesives.
In addition to wetting, the bond between adhesive and substrate will be strong if there is a decrease in the free energy as a result of combining the two. The free energy change will be negative if the heat of mixing, which is positive for most nonpolar and moderately polar pairs, is small. Since the square of the difference of the solubility parameters is proportional to the heat of mixing, it is highly desirable that the solubility parameters for adhesive and substrate be approximately equal.
It follows from the above that poly(vinylmethylether), PVME, is a potentially attractive component in an adhesive formulated to bond to polyolefin surfaces. The critical surface tension of PVME is 29 dynes/cm and the solubility parameter is in the range 8.5-12.7. Comparison of these values with those of polyethylene and polypropylene given above suggests that PVME can wet the polyolefin surface and that the heat of mixing will be small. Both points suggest that PVME could be useful in polyolefin hot-melt adhesives to form strong bonds between the polyolefin and various substrates.
Poly(vinylmethylether) also possesses several other interesting properties that are relevant to its use in adhesive formulations. While atactic, isotactic, and syndiotactic forms of the polymer are known, it is the atactic (amorphous) polymer that is of greatest interest in adhesives. This amorphous material is a very viscous resin (typical number average molecular weight in the neighborhood of 5,000-100,000) and it has a wide solubility range, e.g., it is soluble in water, as well as organic solvents like toluene. This property may be exploited by using PVME as a compatibilizing agent for dissimilar adhesive components. The hydrophilicity of PVME suggests that surface water and moisture should not significantly interfere with bonding. The presence of pendant, polar ether groups provides sites for favorable interaction with other adhesive components and polymer surfaces.
The literature describing the bonding of polyolefins is extensive. Substrate surface treatment and adhesive formulation are the two general approaches the patent literature has taken to improve adherence. In the latter art both hot-melt and pressure-sensitive adhesives are taught. The literature teaching the use of poly(vinylalkylether) as an adhesive component is also broad. For example, a hot-melt adhesive containing poly(vinylmethylether) used to laminate such materials as Kraft paper, burlap, cellophane, cellulose, glass, iron and tin is taught in U.S. Pat. No. 2,970,974.
Conventional adhesives that contain a pitch and a poly(vinylalkylether) fluidized by a solvent, which are considered to operate synergistically, are taught as useful to bind polyolefins in U.S. Pat. No. 3,409,498. The patent further relates that although a broad range of pitch sources produce a suitable adhesive, "Asphalts and other bituminous substances do not produce the same adhesive effects . . . ", which quotation appears at column 3, lines 15-17. Poly(vinylmethylether) is specifically taught as useful in the patent.
Hot-melt, pressure sensitive adhesive compositions consisting of a primary polymer which is a solid, homogeneous and essentially random copolymer of styrene and isobutylene and a secondary polymer which inter alia, can be a poly(vinylalkyl ether) are disclosed in U.S. Pat. No. 3,644,252. The Examples teach their use for adhering polyester.
Now it has been found that by combining poly(vinylmethylether) with a thermoplastic resin and, optionally, a tackifier resin, hot-melt adhesives for polyethylene, polypropylene, and ethylene-propylene copolymers can be formed which bond these substrates strongly and in which the adhesive strength of the bond between adhesive and substrate is generally greater than the cohesive strength of the adhesive.